Yup, chamber-blocking. At the moment the enemy releases his attack, you need to chamber yours in the correct direction. Chambering is the process of moving your weapon to a ready-to-strike position. Pressing LMB is chambering and releasing LMB is striking.
Chamber-blocking is a bit difficult because you need to get both direction and timing right, but it pays off because if you manage to pull it it leaves you ready to strike and the enemy blocked-staggered, tough he can still block your attack if he's fast enough - I once had a chain of 4 or 5 chamber-blocks in the melees...
The direction is the same as with manual blocking - if the enemy strikes you from your left you need to chamber left etc. I believe stab and overhead chambering can both block stab and overhead attacks but I haven't confirmed it yet.
Anyways, weapons that cannot be used to block are great for chamber-blocking because they are usually very fast. You can also chamber-block with your fists - I once chamber-blocked a couched lance with my bare hands in a tourney, just for fun...